Credit: X-ray: NASA/CXC/JHUAPL/R.McNutt et al;
Scientists using NASA's Chandra X-ray Observatory have made the first
detections of X-rays from Pluto. These observations offer new insight
into the space environment surrounding the largest and best-known object
in the solar system’s outermost regions.
While NASA's New Horizons spacecraft was speeding toward and beyond
Pluto, Chandra was aimed several times on the dwarf planet and its
moons, gathering data on Pluto that the missions could compare after the
flyby. Each time Chandra pointed at Pluto – four times in all, from
February 2014 through August 2015 – it detected low-energy X-rays from
the small planet.
Pluto is the largest object in the Kuiper Belt, a ring or belt
containing a vast population of small bodies orbiting the Sun beyond
Neptune. The Kuiper belt extends from the orbit of Neptune, at 30 times
the distance of Earth from the Sun, to about 50 times the Earth-Sun
distance. Pluto's orbit ranges over the same span as the overall Kupier
Belt.
"We've just detected, for the first time, X-rays coming from an
object in our Kuiper Belt, and learned that Pluto is interacting with
the solar wind in an unexpected and energetic fashion,” said Carey
Lisse, an astrophysicist at the Johns Hopkins University Applied Physics
Laboratory (APL) in Laurel, Maryland, who led the Chandra observation
team with APL colleague and New Horizons Co-Investigator Ralph McNutt.
“We can expect other large Kuiper Belt objects to be doing the same."
The team recently published its findings online in the journal
Icarus. The report details what Lisse says was a somewhat surprising
detection given that Pluto – being cold, rocky and without a magnetic
field – has no natural mechanism for emitting X-rays. But Lisse, having
also led the team that made the first X-ray detections from a comet two
decades ago, knew the interaction between the gases surrounding such
planetary bodies and the solar wind – the constant streams of charged
particles from the sun that speed throughout the solar system -- can
create X-rays.
New Horizons scientists were particularly interested in learning more
about the interaction between the gases in Pluto's atmosphere and the
solar wind. The spacecraft itself carries an instrument designed to
measure that activity up-close – the aptly named Solar Wind Around Pluto
(SWAP) – and scientists are using that data to craft a picture of Pluto
that contains a very mild, close-in bowshock, where the solar wind
first "meets" Pluto (similar to a shock wave that forms ahead of a
supersonic aircraft) and a small wake or tail behind the planet.
The immediate mystery is that Chandra's readings on the brightness of
the X-rays are much higher than expected from the solar wind
interacting with Pluto's atmosphere.
"Before our observations, scientists thought it was highly unlikely
that we'd detect X-rays from Pluto, causing a strong debate as to
whether Chandra should observe it at all," said co-author Scott Wolk, of
the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
"Prior to Pluto, the most distant solar system body with detected X-ray
emission was Saturn's rings and disk."
The Chandra detection is especially surprising since New Horizons
discovered Pluto's atmosphere was much more stable than the rapidly
escaping, "comet-like" atmosphere that many scientists expected before
the spacecraft flew past in July 2015. In fact, New Horizons found that
Pluto's interaction with the solar wind is much more like the
interaction of the solar wind with Mars, than with a comet. However,
although Pluto is releasing enough gas from its atmosphere to make the
observed X-rays, in simple models for the intensity of the solar wind at
the distance of Pluto, there isn't enough solar wind flowing directly
at Pluto to make them.
Lisse and his colleagues – who also include SWAP co-investigators
David McComas from Princeton University and Heather Elliott from
Southwest Research Institute – suggest several possibilities for the
enhanced X-ray emission from Pluto. These include a much wider and
longer tail of gases trailing Pluto than New Horizons detected using its
SWAP instrument. Other possibilities are that interplanetary magnetic
fields are focusing more particles than expected from the solar wind
into the region around Pluto, or the low density of the solar wind in
the outer solar system at the distance of Pluto could allow for the
formation of a doughnut, or torus, of neutral gas centered around
Pluto's orbit.
That the Chandra measurements don't quite match up with New Horizons
up-close observations is the benefit – and beauty – of an opportunity
like the New Horizons flyby. "When you have a chance at a once in a
lifetime flyby like New Horizons at Pluto, you want to point every piece
of glass – every telescope on and around Earth – at the target," McNutt
says. "The measurements come together and give you a much more complete
picture you couldn't get at any other time, from anywhere else."
New Horizons has an opportunity to test these findings and shed even
more light on this distant region – billions of miles from Earth – as
part of its recently approved extended mission to survey the Kuiper Belt
and encounter another smaller Kuiper. It is unlikely to be feasible to
detect X-rays from MU69, but Chandra might detect X-rays from other
larger and closer objects that New Horizons will observe as it flies
through the Kuiper Belt towards MU69.
Belt object, 2014 MU69, on Jan. 1, 2019.
The Johns Hopkins University Applied Physics Laboratory (APL) in
Laurel, Maryland, designed, built, and operates the New Horizons
spacecraft and manages the mission for NASA’s Science Mission
Directorate. NASA's Marshall Space Flight Center in Huntsville, Alabama,
manages the Chandra program for NASA's Science Mission Directorate. The
Smithsonian Astrophysical Observatory in Cambridge, Massachusetts,
controls Chandra's science and flight operations.
An interactive image, a podcast, and a video about the findings are available at: http://chandra.si.edu
For more Chandra images, multimedia and related materials, visit: http://www.nasa.gov/chandra
Media contacts:
Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu
